Ultra Wide-Band (UWB) technology based on Multi-Band OFDM (MB-OFDM) is considered the next likely industry standard for the near-distance high data-rate communication technology. Unlike conventional licensed wireless services such as cellular phone, broadcast TV, satellite TV, earth surveillance satellite, weather and airborne radar, UWB radio is an unlicensed radio technology using the frequency range of 3.1 to 10.6 GHz which overlays with conventional service bands. In order to eliminate the possibility of UWB interference victimizing these incumbent services, the revised Federal Communications Commission (FCC) Rules limits the transmission power level of UWB to be below −41.25 dBm/MHz within the 3.1 GHz to 10.6 GHz frequency range.
In OFDM, each tone (subcarrier) is modulated by quadrature amplitude modulation (QAM) or quaternary phase shift keying (QPSK). When a subcarrier is modulated, its bandwidth expands from a single frequency (bandwidth zero) to a non-zero bandwidth. OFDM includes a number of such modulated tones at pre-defined frequency intervals.
Currently the UWB industry targets 3.1 to 5 GHz because this more limited frequency range is better served by existing technology. The broad definition of the UWB band has been officially accepted by other regulatory authorities including the European Union and authorities in Asia. The emission level limitations are subject regional regulatory approval.
FIG. 1 illustrates the spectrum of an OFDM transmission having 128 tones. Each tone occupies 4.125 MHz space for a total active bandwidth of 528 MHz. This MB-OFDM transmission occupies this 528 MHz band within the 3.1 to 10.6 GHz frequency range. At a carrier frequency of 3.8 GHz the allowed transmission power is:PT=−41.25 dBm+10 log 528 dBmPT=−41.25 dBm+27.23 dBmPT=−14.02 dBm
FIG. 1 also illustrates two sidebands of unwanted interference 101 and 102 accompanying the transmission band 100. These two sidebands 101 and 102 are only 20 dB below the magnitude of the OFDM spectrum bandwidth signals 100. They represent significant potential interference to victimize potential competing services in the frequency ranges 101 and 102.
The users of the bands protected by ITU-R Recommendation and Regional Radio Law have raised strong objection to this transmission power level. These users include radio astronomy groups using the bands 3269 to 3267 MHz, 3332 to 3339 MHz and 3345.8 to 3352.5 MHz. Only those radio astronomy bands below 4.7 GHz are listed because the 3.1 to 4.7 GHz band is of greater commercial interest due to the accompanying impact on chip manufacturing cost.
In MB-OFDM each OFDM symbol consists of 128 subcarriers and thus the bandwidth of each symbol is 500 MHz. Three such bands are placed in-between 3.1 and 4.9 GHz and symbols are transmitted using these bands sequentially. A radio system in the 3.1 to 5 GHz band will be hit by at least one of the MB-OFDM bands. Thus some means of protecting the potentially victimized bands is needed.
There are a number approaches proposed to comply with the current and future requirements to protect specific bands. One obvious solution is to notch out the specific bands by narrow-band filters. However, the design of narrow-band RF-band notch filters is a challenging problem and can only be achieved at a high chip cost. Using a conventional narrow-band filter design is far from simple. The center notch frequency of the narrow band filter must be adjustable according to the regional (e.g. Europe and Japan) spectrum use.
Another approach possible only in OFDM stops the transmission of a number of the OFDM sub-carriers particularly where these ODFM sub-carriers occupy frequencies within the interference band. These OFDM sub-carriers are referred to as tones and are synonymous with spectral points on the QAM constellation diagram space. OFDM communicates information bits as a collection of modulated narrow-band tones using Fast Fourier Transform. In MB-OFDM each tone occupies a bandwidth of about 4 MHz. In order to prevent the interference within a radio astronomy bandwidth of 7 MHz, two tones located in the potentially victimized frequency band are not transmitted. These are called zeroed tones. Stopping transmission of a number of the OFDM sub-carriers is potentially more flexible than notching out the specific bands by narrow-band filters. This narrow-band notch filtering is realized by digital signal processing control of OFDM modulation. This is more attractive from the viewpoint chip implementation complexity and cost.
Each tone or subcarrier is a single frequency having a bandwidth of zero. When a subcarrier is amplitude modulated in QAM (Quadrature Amplitude Modulation) or PSK (Phase shift Keying) the resulting bandwidth is non-zero. Applying amplitude modulation causes each subcarrier to carry information. In 64 QAM, each subcarrier carries 6 bits. Eliminating a subcarrier in 64 QAM results in 6 bits being removed from one symbol.
There is a question whether stopping transmission of a number of the OFDM sub-carriers can completely solve the interference problem. Consider an example involving interference to the radio astronomy band of 7 MHz.
Elimination of the interference to a specific band is a problem of both bandwidth and attenuation level. Under the current Japan Radio Law, the acceptable unintentional ambient radiation level is −64.3 dBm/MHz. This is the peak signal power level, but in the 1 MHz bandwidth range the peak and average power levels are almost identical. Because the in-band (3.1 to 10.6 GHz) radiation of the UWB signal is limited to −41.3 dBm/MHz according to FCC Rules, one way for the UWB transmitter to coexist with the radio astronomy service is to lower the UWB interference in the radio astronomy band to the ambient noise level. This would require attenuation from 64.3 dBm/MHz to 41.25 dBm/MHz, or 23 dB attenuation to the transmitted MB-OFDM signal at the band location.
Because the radio astronomy band is 7 MHz wide, at least two OFDM tones must be eliminated (zeroed). FIGS. 2a, 2b, and 2c illustrate the effects of eliminating two, four and sixteen tones respectively. Eliminating two tones (FIG. 2a) reduces interference in the victimized band by approximately 10 dB; eliminating four tones (FIG. 2b) results in no further significant interference reduction. FIG. 2b shows an expanded frequency range for more clarity in the interference band. It is clear that the simple zero-toning method requires far more tones to be eliminated. Because the interference level is dependent on the data modulated on the tones surrounding the interference band, the required number of zeroed tones is likewise dependent on the modulated data. As illustrated in FIG. 2c, even increasing the number of zeroed tones to sixteen does not provide the required 23 dB attenuation at the designated band location. When sixteen tones eliminated and the target elimination bandwidth is 7 MHz, a full 64 MHz of OFDM signal bandwidth is sacrificed. Achieving the required 23 dB elimination would thus cause excessive degradation of the spectrum usage and lowers the communication throughput.
Summarizing the concerns and challenges described, it is clear that a more effective solution is needed that achieves the required level of interference suppression using the minimum number of sacrificed tones. This optimization task also involves issues of cost and complexity related to the hardware implementation.